The use of two-component silicone rubbers which vulcanize at room temperature as a material for making flexible female molds is known (see, W. Noll, Chemie und Technologie der Silicone [Chemistry and technology of silicones], Verlag Chemie, Weinheim, 2nd edition 1964, pp. 339-340). With the aid of such molds, even models having highly complex surface structures can be reproduced and duplicated using various materials. In addition to the high flexibility and mechanical strength, which enable demolding by vigorous stretching or even by glove-like turning inside out, the easy workability and high accuracy of reproduction in conjunction with the strongly adhesive effect to most of the currently used reproduction materials form the basis for the leading position of the RTV-2 silicone rubbers as mold materials in the industrial and artistic field of duplicating objects.
One of the most important and most frequently utilized fields of application for flexible female molds made of RTV-2 silicone rubber is the manufacture of small series of moldings from organic resins, in particular unsaturated polyester and polyurethane resins. Small series of moldings are required where, the existence of numerous different designs, each requires only relatively low production runs, so that it becomes uneconomic for molds for casting or injection molding to be made from e.g. metal, because of their high costs, e.g. in the manufacture of prototypes, bathroom and kitchen furniture panels, period furniture, picture frames and mirror frames, promotional material, souvenirs and the like. Keeping the unit costs as low as possible requires the highest possible molding frequency, i.e. number of reproductions per mold.
Unfortunately, flexible molds made of RTV-2 silicone rubber have a limited service life. Apart from purely mechanical damage caused by the demolding procedure, physical and chemical effects will give rise, depending on the degree to which the mold surface is structured, on the type and composition of the reproduction material and on the number of casts made per day, to an increasing loss in release properties. This manifests itself by an increasing force required for demolding, until finally the casting may partially adhere to the mold, so that demolding is no longer possible without damaging the mold surface. The mold has reached the end of its service life.
In unsaturated polyester resins containing styrene, the styrene penetrates the boundary layer of the rubber mold and partially polymerizes within the silicone rubber matrix to give polystyrene, with the formation of an interpenetrating network of polydimethylsiloxane and polystyrene, so that the proportion of polydimethylsiloxane per unit volume decreases. This results in embrittlement and in a reduction of the release properties of the mold surface. The mold fails either as a result of mechanical damage, since the embrittled areas of the mold are no longer sufficiently flexible for the mold to be stretched during demolding of the resin castings, or alternatively as a result of adherence of a casting to a larger or smaller area, owing to by now inadequate release properties of the mold surface.
In contact with polyurethane resins the silicone rubber will be chemically attacked by the isocyanate component, i.e. the NCO will react with SiOH functions still present, e.g. from the hydrolysis of excess crosslinker, to form copolymer structures, as a result of which the surface of the mold similarly successively loses its release properties with respect to the casting resin, until adherence of a casting results.
The higher the reaction temperature of the casting resin, and the longer the rubber mold is subjected thereto, the heavier it will suffer from chemical attack, i.e. the lower will be the number of casting per mold.
There have been no attempts to extend the service life of flexible molds made of RTV-2 silicone rubber, by applying an additional release agent layer which has the effect of repelling the organic resin. Such external release agents based on waxes, low- and high-molecular weight silicone oils or silicone resins, or perfluorinated hydrocarbons are known (e.g. Beck and Smith, EP 404325, and East, U.S. Pat. No. 5,380,478), and some of them are available from specialized manufacturers such as, e.g., Wurtz, Acmos and Arti. They have distinct drawbacks however, since they can be used only as solvent-containing preparations in a thin layer and virtually all of the solvents suitable for this purpose caused marked swelling of the vulcanized silicone rubber. It is necessary to wait after application of the above release agents, until the solvent has completely volatilized from the vulcanized rubber, since molds which still contain solvent suffer noticeable deterioration in terms of their stability with respect to organic resins. Another disadvantage is that these release agents must, as a rule, be freshly applied after each demolding operation, since they are partially carried away with the casting and this, in turn, may have an adverse effect on the paintability of the latter. Furthermore, the release agent tends to accumulate in poorly accessible corners and under cuts resulting in decreased reproduction accuracy. The additional work required further increases the unit costs.
Attempts have been made to avoid the drawbacks of external release agents by using internal releasing agents. Such release agents, e.g. higher-molecular weight silicone oils or hydrocarbons, exhibit a certain incompatibility with the vulcanized silicone rubber and therefore migrate to the mold surface, where they form an external release agent film. While they do not have the drawback of requiring repeated application and of containing solvents which cause swelling, they do suffer from the other abovementioned drawbacks of external release agents.
Since the attack on the silicone rubber surface, whether it be physical or chemical, is usually associated with a swelling process of the vulcanized material, attempts were further made to increase the resistance to casting resin by optimized formulations of the silicone rubber molding compositions in terms of polymer/crosslinker content, filler type and filler content, and catalyst type and catalyst content (e.g. Smith, EP 586153; Frances, EP 378952; Gibard, EP 010478).
While all the methods mentioned are able to improve, to a certain extent, the resistance of molds made of RTV-2 silicone rubber with respect to organic resins, they are unsatisfactory with regard to the degree of the improvement and owing to drawbacks with regard to processing effort, reproduction accuracy or further processing of the resin moldings produced.